C3 or C-3 can refer to:

• Clinical Cell Culture, a medical technology company.
• C3, a pathway for carbon fixation in photosynthesis.
• C3 plants, plants that primarly use the C3 pathway.
• HMS C3 , a C-class submarine of the Royal Navy.
• USS C-3, a C-class submarine of the United States Navy.
• USS Baltimore (C-3), a protected cruiser of the United States Navy.
• C3, an international standard paper size (324×458 mm), defined in ISO 216.
• C3, a form of plastic explosive.